Environmental and Systemic Exposure Assessment for Green Sturgeon Following Application of Imidacloprid for the Control of Burrowing Shrimp in Willapa Bay, Washington

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The neonicotinoid insecticide imidacloprid (IMI) has been proposed for the control of burrowing shrimp on oyster beds in Willapa Bay, Washington. Use of IMI may be denied if it is shown that ESA-listed green sturgeon are adversely affected as a result of exposure to the compound. Green sturgeon were found to feed opportunistically on IMI-impaired shrimp following test applications of the insecticide. Foraging activities could result in exposure to IMI in sediment pore water and from the consumption of contaminated shrimp. An exposure characterization revealed maximum IMI residue concentrations of 27.8 μg L<super>-1</super> in pore water and 31.4 μg kg<super>-1</super> in shrimp. Results from modeling branchial and dietary uptake indicated that pore water exposure would account for most of the systemic absorption of IMI. The 96-h median lethal toxicity (LC<sub>50</sub>) for surrogate juvenile white sturgeon was 124 mg L<super>-1</super>, indicating sturgeon do not possess extreme sensitivity to IMI. Controlled exposures to white sturgeon of maximum IMI field concentrations did not result in overt effects, but showed relatively long persistence of the chemical in plasma. Toxicokinetic studies were conducted with rainbow trout (RBT) to describe the disposition and elimination of IMI in fish. Results suggest no dose-dependence on kinetics following interarterial bolus injections of a low (44.7 μg kg<super>-1</super>) or high (250.0 μg kg<super>-1</super>) dose of IMI. Mean values for the steady-state volume of distribution (<italic>V</italic><sub>ss</sub>) were 1561.9 and 1922.5 mL kg<super>-1</super> for the low and high dose groups, respectively. Mean whole body clearance (<italic>CL</italic><sub>B</sub>) was 20.5 mL h<super>-1</super> kg<super>-1</super> for both groups. The elimination half-life (t<sub>1/2</sub>) was 65.6 and 68.2 h for the low and high dose groups, respectively. Mass-balance analysis revealed that branchial clearance accounted for almost 50% of <italic>CL</italic><sub>B</sub>. Measured tissue:plasma IMI residue concentration ratios in RBT and white sturgeon were comparable, suggesting that both species handle the chemical in a similar manner. The hazard quotient (HQ), the ratio of the maximum pore water concentration over the LC<sub>50</sub> value, was two orders of magnitude below the threshold for potential effects. This implies that green sturgeon would be at minimal risk from exposure to IMI in Willapa Bay.